<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>css 变量</title>
    <style>
        html{
            --primary_color: red;
        }

        div{
            width: 200px;
            height: 200px;
            background-color: var(--primary_color, green);
        }
    </style>
</head>
<body>
    <div>css 变量</div>
</body>
</html>